1. Field of the Invention
The present invention relates to an image-forming apparatus and a fixing unit used in electrophotographic copying machines, printers, etc.
2. Description of the Related Art
Heretofore, an image-forming apparatus is widely used in electrophotographic copying machine and printers, in which an electrostatic latent image is formed on a photoreceptor, it is developed with a dry toner, and the resulting toner image is electrostatically transferred onto a recording medium and fixed thereon. In the image-forming apparatus, however, recording paper, a type of recording medium could not be kept in tight contact with the photoreceptor owing to its surface roughness, and often forms some uneven gaps between them, whereby the transfer field around the recording paper will be disordered and the toner grains thereon will produce Coulomb repulsion. One problem with it is that the image formed on the recording paper is thereby often disordered to cause image deletion or noise.
To solve the problem, some advanced types of image-forming apparatus have been proposed. In one type of the apparatus for image formation, plural toner images of different colors are electrostatically transferred onto an intermediate transfer member to form a multi-layer toner image thereon, then the multi-color multi-layer toner image is fed on the member, and the thus-fused multi-layer toner image is transferred and simultaneously fixed on a recording medium in a predetermined simultaneous transfer and fixing zone to thereby form a color image on the recording medium, and in another type, the toner image formed on an endless photoreceptor is fused thereon, and is then transferred and fixed on a recording medium.
In these types of image-forming apparatus, the toner image is non-electrostatically transferred onto the recording medium and will be therefore free from the problem of image quality deterioration noted above. In these, however, the part of the intermediate transfer member and that of the photoreceptor which are directly or indirectly kept in contact with a photoreceptor cleaner and a development unit must be cooled to a temperature not higher than melting temperature of the toner, in order that the toner in the photoreceptor cleaner and in the development unit is prevented from melting therein. To meet the requirement, another type of image-forming apparatus is disclosed, for example, in Japanese Patent Laid-Open Nos. 12988/1984 and 95568/1987, in which a forcedly or spontaneously cooling member is disposed in the traveling route of the intermediate transfer member and the photoreceptor along which the intermediate transfer medium with a toner image formed thereon passes through the simultaneous transfer and fixing zone to reach the photoreceptor cleaner and the development unit, or the traveling route of the two is prolonged to ensure a prolonged cooling time for the two.
However, in the image-forming apparatus of that type, the intermediate transfer medium and the photo or cooled to a temperature not higher than the melting temperature of the toner must be re-heated up to the temperature necessary for transfer and fixation in every image formation cycle, and this carries another problem as requiring large power.
To solve the problem Japanese Patent Publication No. 36341/1983 discloses a modified image-forming apparatus in which the heat capacity of the intermediate transfer member is limited to at most 3.1xc3x971013 cal/cm2xc2x7xc2x0 C. to thereby reduce the heat energy necessary for heating the member in every image formation cycle. To reduce the heat capacity of the belt in the apparatus, in general, the belt is thinned. However, the belt could not be thinned so much so as not to reduce its strength. The belt not so much thinned to have a practicable thickness could not attain a significant result. A metallic belt may be employed to reduce its thickness without lowering its strength, but this is electroconductive and is therefore problematic in that it could not apply to an ordinary electrostatic transfer system widely used for forming a toner image on an intermediate transfer member. Another method may be employed that includes applying pressure to an intermediate transfer member for forming a toner image thereon. However, this is problematic in that, when high pressure is applied to the member, the apparatus shall be complicated and large-sized.
On the other hand, Japanese Patent Laid-Open No. 298777/1992 discloses an image-forming apparatus in which the belt running toward the image-forming zone and the belt running toward the simultaneous transfer and fixing zone are contacted with each other for heat exchange therebetween in the region between the image-forming site of a belt-like image bearing member and the simultaneous transfer and fixing zone, to thereby reduce the energy loss in that region of the apparatus.
In the image-forming apparatus of that type, however, the two belts are directly contacted with each other and therefore require an additional urging member to press the belts against each other. In this, the urging member is provided above the surface of the upper belt, and it will scratch the surface of the belt to worsen the quality of the images formed. In addition, since the belts are contacted with each other, they must be elastic and require a space in which they are warped before and after they are contacted with each other. This is problematic as the apparatus must be large-sized. Moreover, since the belts are insulators, they are electrostatically charged when contacted with each other. Therefore, their adhesion to each other will be good to enhance beat exchange between them, but the belts could not well run and will often cause image registration failure and other image defects. This is still another problem with the image-forming apparatus.
To solve these problems, a heat conductor may be disposed between a part of the belt running toward the image-forming zone and a part thereof running toward the simultaneous transfer and fixing zone for attaining heat exchange between the two parts of the belt. However, merely disposing such a heat conductor between the parts of the belt would average the temperatures of those parts, but could not attain sufficient heat energy exchange between them, and the heat exchange efficiency of this system will be low.
To solve this problem, Japanese Patent Laid-Open No. 213977/1998 discloses an image-forming apparatus in which a heat circulator is disposed between a part of the belt running toward the image-forming zone and a part thereof running toward the simultaneous transfer and fixing zone. The heat circulator is directly contacted with the two parts of the belt to attain heat exchange between them, and this prevents the tempts of the two parts of the belt from being averaged, and therefore realizes high heat exchange efficiency. Precisely, the heat circulator includes plural metal members aligned perpendicularly to the traveling direction of the belt both on the side of the belt that runs toward the image-forming zone and on the other side thereof that runs toward the simultaneous transfer and fixing zone, and heat pipes that connect these plural metal members. In his, the neighboring plural metal members are bonded to each other via a heat-insulating spacer therebetween.
However, the heat circulator carries some problems mentioned below. Specifically, in the heat circulator, the neighboring plural metal members are connected with each other via a heat-insulating spacer therebetween. Therefore, in order that the heat circulator has satisfactory mechanical strength, the side surface area of each metal member must be enough to connect the neighboring metal members to each other, and too small metal members could not satisfy the requirement. As a result, it is inevitable to enlarge the heat circulator, and cutting the cost for producing the heat circulator will be difficult. In addition, in order that the heat circulator attains efficient heat exchange between the two parts of the running belt while it is kept in contact with them, the heat circulator must be well polished to have an accurately curved surface with no roughness. However, since the metal members are connected to construct the heat circulator, a difference in level between the connected metal members will he inevitable. Therefore, it will take a lot of time and will be expensive to accurately construct the heat circulator, and the cost for producing the heat circulator will be high.
FIG. 15 shows one example of the conventional heat circulator to be in an image-forming apparatus.
As in FIG. 15, the conventional heat circulator includes a heat-receiving part 11 composed of plural metal members 22 that are aligned in parallel with each other but perpendicularly to the belt-traveling direction A, a heat-radiating part 12 also composed of plural metal members 22 that are aligned in parallel with each other but perpendicularly to the belt-traveling direction A, and plural, closed cylindrical heat pipes 13 that connect the metal members 22 of the heat-receiving part 11 to those of the heat-radiating part 12.
The heat-receiving part 11 is disposed at the position at which it is contacted with a hot part 21a of the belt moving and circulating in the direction A, and this receives the heat from the hot belt part 21a, and transfers the heat to the heat-radiating part 12 disposed at the position at which it is contacted with a low-temperature part 21b of the other belt, via the heat pipes 13.
The metal members 22 are made of a heat-conductive metal such as aluminium, copper or their alloy, but it is said that an extrusion molding of aluminium or an aluminium alloy is preferred for them as being inexpensive and well workable.
FIG. 16A and FIG. 16B are to show the outline of the metal members for the heat circulator shown in FIG. 15; and FIG. 17 is a perspective view of one metal member shown in FIG. 16A and FIG. 16B.
As in FIG. 16A and FIG. 16B, the heat-receiving part 11 and the heat-radiating part 12 each include plural metal members 22 aligned in parallel with each other but perpendicularly to the belt-traveling direction A, and every metal member 22 has a through-hole 23 that runs in the direction of the width of the belt. The both side surfaces of every metal member 22 are worked to have a groove 24 that runs in the direction of the width of the belt.
A straight part 13a of the heat pipe 13 is inserted into the through-hole 23. Every through-hole 23 is so formed that it is spaced from the back surface of each metal member 22 by the same depth d. With all through-holes 23 being so disposed, the straight part 13a of every beat pipe to be inserted into each through-hole 23 may have the same size, and this increases the working efficiency in fabricating the heat circulator.
The grooves 24 are to enhance the heat insulation between the neighboring metal members 22 and to reduce the weight of the heat circulator. The neighboring metal members 22 are spaced from each other by a distance of from 0.05 mm to 2.0 mm therebetween, and are integrally bonded to each other via brazed joints 25.
As in FIG. 17, the brazed joints 25 are formed partly only at the both edges of the metal member, concretely around the upper edges and the lower edges of the side surface of the metal member not brought into contact with a belt, and the brazed area will be about 40 mm2 or so in total.
FIG. 18 shows an outline of the constitution of an image-forming apparatus with the conventional heat circulator built therein.
In the image-forming apparatus illustrated, a toner image is formed on the surface of a belt 21 in the image-forming region (not shown), and the belt 21 thus carrying the toner image formed thereon moves in the direction of the arrow A. When the belt 21 has reached the beat-radiating part 12 of a heat circulator 20, it is heated by the plural metal members 22 in the heat-radiating part 12, then further heated by a heating plate 9, and thereafter still further heated under pressure between a heating roll 2 and a pressure roll 3 in a subsequent simultaneous transfer and fixing zone F. Through the process, the toner image thus carried by the belt 21 is transferred and fixed on a recording medium
The belt 21 thus heated in the simultaneous transfer and fixing zone F and therefore having a high temperature is then brought into contact with the plural metal members 22 in the heat-receiving part 11 of the heat circulator 20, and is thus gradually cooled through heat exchange between them. The thus-cooled belt 21 further circulates and moves in the direction of the arrow A, and then a toner image is again formed on its surface in the image-forming region (not shown). With the toner image formed thereon, the belt 21 further runs toward the heat-radiating part 12 of the heat circulator 20.
On the other hand, the plural metal members 22 in the heat-receiving part 11 that have received the heat from the belt 21 transfer the heat to the plural metal members 22 in the heat-radiating part 12 via the heat pipes 13, and the plural metal members 22 in the heat-radiating part 12 transfer the heat to the belt 21 having a low temperature.
The neighboring metal members 22 in the heat-receiving part 11 and those in the heat-radiating part 12 are thermally insulated from each other. Therefore, the high-temperature metal members in the heat-receiving part 11 are kept still having a high temperature also in the heat-radiating part 12, and the two parts realize high heat-exchange efficiency.
Back surfaces 11b, 12b of the metal members 22 in the heat-receiving part 11 and the heat-radiating part 12 that are not brought into contact with the belt 21 are finished to be nearly flat, as in FIG. 16A; but outer surfaces 11a and 12a thereof that are brought into contact with the belt 21 are cut and polished to have a gently curved profile having a radius of curvature of from 300 mm to 2000 mm or so in order to ensure good contact between the surface and the belt 21. While their surfaces are cut and polished, the metal members 22 are so controlled that the surface level difference between the neighboring metal members 22 could be at most 100 xcexcm. If the surface level difference is larger than 100 xcexcm, the belt 21 could not be well kept in contact with the metal members 22, and if so, the heat-exchange efficiency between them will greatly lower. Therefore, it is extremely important to minimize the surface level difference between the neighboring metal members 22.
However, while their surfaces are cut and polished, the metal members must be prevented as much as possible from being thermally expanded owing to the temperature increase during the process of working them and must be prevented as much as possible from being deformed owing to the stress that they will receive while worked. The working process requires the utmost care, and will often take one full day or so. For these reasons, constructing the heat circulator takes a lot of time and much labor, and the heat circulator thus constructed is expensive.
The problems noted above also apply to the case of disposing the heat circulator in a fixing unit to be used in electrophotographic image formation. For example, a fixing unit of a type in which a recording medium with a toner image formed thereon is, while put on a circulating endless belt-type fixing member, passed through a fixing zone including a heating roll and a pressure roll so that the toner image is fixed on the recording medium under heat and pressure in that zone will carry various problems such as those noted above, when it is provided with a heat circulator of a type that is kept in contact with the fixing member at positions both upstream and downstream the fixing zone so that the heat circulator can transfer the heat of the fixing member downstream of the fixing zone to the part of the fixing member upstream of the fixing zone.
The present invention has been made in view of the above circumstances, and provides an image-forming apparatus equipped with a heat circulator that achieves high heat exchange efficiency and is inexpensive, and also provides a fixing unit quipped with such a heat circulator.
Specifically, the first aspect of the invention is to provide an image-forming apparatus which includes an endless image bearing member that moves while carrying a toner image formed thereon; a heating unit disposed in the traveling route of the image bearing member; a first metal member to come into contact with the image bearing member at a position downstream of the heating unit in the traveling direction of the image bearing member; a second metal member to come into contact with the image bearing member at positions downstream of the first metal member and upstream of the heating unit in the traveling direction of the image bearing member; and plural heat-transporting members aligned in the traveling direction of the image bearing member and clamped between the first and second metal members.
The second aspect of the invention is to provide a fixing unit which includes a movable endless belt; a heating unit to come into contact with the belt to heat it; a first metal member to come into contact with the belt at a position downstream of the heating unit in the traveling direction of the belt; a second metal member to come into contact with the belt at positions of downstream of the first metal member and upstream of the heating unit in the traveling direction of the belt; and plural heat-transporting members aligned in the traveling direction of the belt and clamped between the first and second metal members.
The third aspect of the invention is to provide a heat circulation system which includes a movable endless belt; a heating unit to come into contact with the belt to heat it; a pair of metal members that come into contact with the belt at positions upstream and downstream of the heating unit in the belt traveling direction; and plural heat-transporting members to connect the metal members, and in which the heat of the endless belt downstream the belt of the heating unit in the traveling direction is circulated to the part of the belt upstream of the heating unit.